class(*), allocatable stole my sequence type

[ivan-pi]

Given a sequence type stored in an unlimited polymorphic variable, the compiler doesn't approve of even a single method to access the data:

! seq_type.f90

type :: seq_type
   sequence
   real :: val
end type

class(*), allocatable :: a

a = seq_type(val=3.0)

select type(a)
type is (seq_type)
    print *, a%val
end select

block
    type(seq_type) :: b
    call move_alloc(from=a,to=b)
    print *, b%val
end block

block
    type(seq_type) :: b
    b = transfer(a, b)
    print *, b%val
end block

end

~> nagfor sequence_upv.f90 
NAG Fortran Compiler Release 7.2(Shin-Urayasu) Build 7203
Error: sequence_upv.f90, line 14: TYPE IS specifies SEQUENCE type SEQ_TYPE
Error: sequence_upv.f90, line 20: Expected an ALLOCATABLE variable for argument TO (no. 2) of intrinsic MOVE_ALLOC
Warning: sequence_upv.f90, line 26: Intrinsic TRANSFER from polymorphic data object might have partially undefined result
[NAG Fortran Compiler error termination, 2 errors, 1 warning]

This is a corner case of the discussions in,

• Class(*) stole my object!
• Data-pointer-object and unlimited polymorphic target

[ivan-pi]

Unfortunately the solution proposed in #345 would not deal with sequence types.

select type can't be used due to (J3/24-007):

C1166 (R1156) The type-spec or derived-type-spec shall not specify a derived type with the BIND attribute or the SEQUENCE attribute.

I believe the only legal solution at the moment is using a pointer to a target:

class(*), allocatable, target :: a
allocate(a,source=seq_type(val=3.0))
block
    type(seq_type), pointer :: b => null()
    b => a
    print *, b%val
end block